Sheet feeding apparatus and method of detecting double feed

ABSTRACT

A sheet feeding apparatus includes a stacker for stacking a sheet; a feeding device for feeding the sheet on the stacker to a predetermined processing position; a drive device for driving the feeding device; a double feed detection device having a sending element and a receiving element arranged at a downstream side of the feeding device for detecting a double feed of the sheet; a sensitivity adjustment device for comparing a detected value of the receiving element with a predetermined reference value for adjusting an output of the sending element; and a control device for controlling a transport speed of the feeding device. The sensitivity adjustment device adjusts the output of the sending element when the control device controls the drive device to stop the sheet or to decelerate the sheet at a predetermined speed.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a sheet feeding apparatus forsequentially separating sheets such as originals on a stacker into asingle sheet and feeding the sheet to a processing platen for reading inother processes. More particularly, the present invention relates to asheet feeding apparatus provided with a double feed detection functionfor detecting a double feed of sheets in a path from the stacker to aprocessing platen.

A conventional sheet feeding apparatus feeds sheets such as originals toa processing platen. A reading apparatus for reading images on thesheets at the processing platen is widely known as a scanner, copier, ora facsimile machine. An apparatus for printing at a processing platen iswidely used as a printer.

It is necessary to accurately separate the sheets into a single sheetand to feed the sheet to the processing platen with proper positioning.Particularly, when sequentially reading a series of originals, it ispossible that the sheet is not fed to the platen, i.e., a non-feed, ortwo or more sheets are fed to the platen, i.e., a double feed, therebycausing an improper process at the processing platen. When printing aseries of originals at the processing platen, an incorrect process mayoccur at the platen due to the non-feed or double feed.

Accordingly, it is necessary to stop a process when the double feed ornon-feed is detected while feeding the sheet from the stacker to theprocessing platen. For example, a sensor is provided for detecting thesheet prior before the processing platen (at an upstream side). When thesheet does not reach a predetermined position for a predetermined amountof time after the sheet is fed from the stacker, it is determined thatthe non-feed occurs and the process at the platen stops.

It is difficult to accurately detect the double feed of the sheets, anda detection element and a judgment circuit are expensive. In a system inwhich scanners or copiers are linked through a network, the double feedof the sheets can cause a serious system error. Accordingly, it isnecessary to accurately detect the double feed with low cost.

Japanese Utility Model (Kokoku) No. 06-49567, and Japanese PatentPublications (Kokai) No. 2000-95390 and No. 2003-176063 disclose devicesfor detecting the double feed using a pair of ultrasonic wave sensors.In the devices, a pair of ultrasonic wave sensors is arranged atopposing positions to sandwich a sheet in a sheet transport path. Asensor on a wave receiving side (wave receiving element) detectsultrasonic waves emitted from a wave sending side (wave sending element)to detect the double fed through attenuation of the ultrasonic wavespermeating the sheet.

FIG. 2 shows an example of a widely known ultrasonic wave sensor fordetecting the sheet. A piezoelectric diaphragm is embedded in a metalcase covering the sensor. A high-frequency voltage is applied to anelectrode formed on the piezoelectric diaphragm, thereby generatingultrasonic waves from a surface of the case. The case is filled with aplastic 13. A wave receiving element has a structure same as that of thewave sending element and receives the ultrasonic waves to oscillate asurface of the case. As a result, the piezoelectric diaphragm fixed tothe case also oscillates to output electrical energy to an externalsource.

The piezoelectric diaphragm may have a variance in a dimension and ashape thereof, and the metal case may have a variance in acharacteristic frequency. Accordingly, in a manufacturing process, it isnecessary to combine the wave sending and wave receiving elements withina tolerable range to achieve accurate detection. Therefore,conventionally, characteristics of each of the elements are measured inthe manufacturing process, so that only the elements within a tolerancelevel are used to produce the ultrasonic wave sensor.

The elements of the conventional ultrasonic wave sensor for detectingthe double-feed of the sheets are measured in the manufacturing processas described above, thereby increasing cost of the elements. When anambient temperature fluctuates or the sensor is deformed upon receivingan impact, it is possible to cause an erroneous detection. Accordingly,it is necessary to select the detection elements with similarcharacteristics within a specific range for the wave sending side andthe wave receiving side, so that the sensors detect the sheets withoutan influence of a temperature or deformation. Accordingly, the apparatustends to be expensive and durability may be compromised.

In view of the problems described above, an object of the presentinvention is to provide a sheet handling apparatus capable of accuratelydetecting a double feed of sheets fed from a stacker to a processingplaten, thereby eliminating an erroneous process at the processingplaten. In the sheet feeding apparatus, detection elements for detectingthe double feed of the sheets are adjusted in a state that the detectionelements are disposed in a transport path, so that a variance incharacteristics between the detection elements at sending and receivingsides is adjusted.

Further objects and advantages of the invention will be apparent fromthe following description of the invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to theinvention, a sheet feeding apparatus includes a stacker for holdingsheets; a feeding device for kicking a sheet from the stacker to apredetermined processing position; and a sending element and a receivingelement disposed at a downstream side of the feeding device fordetecting a double feed of the sheets. The sending and receivingelements are composed of a wave sending element and a wave receivingelement of an ultrasonic wave sensor, and are arranged to face eachother on opposite sides of the sheet. A sensitivity adjustment device isprovided for comparing a detected value of the wave receiving elementwith a predetermined reference value to adjust an output of the wavesending element. A control device of a drive device is provided fordriving the feeding device.

The sensitivity adjustment device adjusts the output of the wave sendingelement when the drive device is stopped or decelerated at apredetermined speed. The sensitivity adjustment device compares theoutput value of the wave receiving element with the predeterminedreference value. According to a result of the comparison, thesensitivity adjustment device increases or decreases, for example,amplitude of high frequency power in a case of the ultrasonic sensor tochange electrical energy supplied to the wave sending element.Accordingly, it is possible to obtain a predetermined output from thewave receiving element. It is possible to stably detect the double feedof the sheets even if detection sensitivity of the wave sending elementand the wave receiving element is degraded due to a temperature changeor aging.

According to the present invention, first and second transport devicesmay be arranged between the stacker and the predetermined processingposition with a gap therebetween. The double feed detection devicehaving the sending element and the receiving element is arranged betweenthe first and second transport devices. The sensitivity adjustmentdevice is disposed for comparing the detected value of the sendingelement to a predetermined reference value and adjusting the output ofthe sending element. A sheet sensor is disposed for detecting that thesheet from the stacker reaches a downstream side of the first and secondtransport devices to send a sheet leading-edge detection signal.According to the sheet leading-edge detection signal, the sensitivityadjustment device adjusts the output of the sending element. Also,according to the sheet leading-edge detection signal from the sheetsensor, the first and second transport devices stop. After thesensitivity adjustment device completes the sensitivity adjustment, thefirst and second transport devices restart to discharge the sheet at ahigh speed.

According to the present invention, a method of detecting a double feedincludes a transport step for transporting a sheet from a stacker to apredetermined sheet processing position; a double feed detection stepfor detecting the double feed of the sheets with an ultrasonic wavesensor comprising a sending element and a receiving element and arrangedbetween the stacker and the sheet processing position; a sensorsensitivity adjustment step for changing an output of the sendingelement by stopping the sheet traveling between the stacker and thesheet processing position or decelerating the sheet at a predeterminedspeed; and a discharge step for moving the sheet after the sensorsensitivity adjustment step or discharging the sheet at a speed higherthan the predetermined speed. In the sensor sensitivity adjustment step,the sheet is nipped by at least two transport devices arranged atupstream and downstream sides in a transport direction of the sheet.

In the invention, the sending element and the receiving element of theultrasonic wave sensor are arranged in a path for transporting thesheets. When detecting the double feed of the sheets, the sensitivityadjustment device compares the output of the receiving element with thepredetermined reference value for adjusting the output of the sendingelement. Accordingly, it is possible to accurately detect the doublefeed through the adjustment of the output of the sending element even ifa fluctuation occurs due to a variance in characteristics of the sendingand receiving side elements, mounting positions of the elements, or anambient temperature.

Accordingly, even when the ultrasonic wave sensor has characteristicschanging easily, a tolerable range of characteristics is widened,thereby reducing manufacturing cost. Further, the output of the wavesending side is adjusted when transport of the sheet such as a testsheet is stopped or the sheet is decelerated to a predetermined speed.Accordingly, it is possible to accurately obtain the output of the wavesending element without an influence of a variance in a sheet transportspeed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a sheet feeding apparatus accordingto the present invention;

FIG. 2 is a schematic view showing a double feed detection devicecomposed of an ultrasonic wave sensor;

FIGS. 3(a) and 3(b) are block diagrams showing a control circuit of thesheet feeding apparatus shown in FIG. 1, wherein FIG. 3(a) shows acontrol circuit for double-feed detection, and FIG. 3(b) shows asensitivity adjustment circuit;

FIGS. 4(a) and 4(b) are graphs showing waveforms of output signals fromthe ultrasonic wave sensor shown in FIG. 2, wherein FIG. 4(a) shows asingle feed and FIG. 4(b) shows a double feed;

FIG. 5 shows a block diagram showing the control circuit of the sheetfeeding apparatus shown in FIG. 1;

FIG. 6 is a flow chart showing an operation of adjusting sensitivity ofthe sheet feeding apparatus shown in FIG. 1;

FIG. 7 is a flow chart showing an operation of detecting the double feedof sheets in the sheet feeding apparatus shown in FIG. 1;

FIG. 8 is a schematic view of an image reading apparatus and an imageforming apparatus with the image reading apparatus as a unit accordingto the present invention;

FIG. 9 is a view showing a sheet feeding unit of the image readingapparatus shown in FIG. 8;

FIG. 10 is a perspective view showing a paper feed stacker of the imagereading apparatus shown in FIG. 9;

FIGS. 11(a) and 11(b) are views showing a drive mechanism of the imagereading apparatus shown in FIG. 9, wherein FIG. 11(a) shows a sheetfeeding unit, and FIG. 11(b) shows a transport unit; and

FIGS. 12(a) to 12(e) are views showing an operation of feeding a sheetin the image reading apparatus shown in FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, preferred embodiments of the present invention will beexplained with reference to the accompanied drawings. The inventionapplies to an apparatus and a method for detecting a double feed of twoor more overlapped sheets before reaching a processing position when thesheets stacked on a stacker in a sheet feeding unit of an image readingapparatus such as a copier, or printer are separated and transported oneby one to the processing position such as an image reading platen, orprinting platen.

FIG. 1 is a schematic view of a sheet feeding mechanism according to anembodiment of the present invention. FIG. 2 is a schematic view of adouble feed detection device composed of an ultrasonic wave sensor.FIGS. 3(a) and 3(b) are circuit diagrams of a control circuit.

As shown in FIG. 1, the sheet feeding apparatus is equipped with astacker 1 for storing sheets; a sheet guide 3 for guiding the sheetsfrom the stacker 1 to the processing platen 2; at least two transportdevices, i.e. first and second transport devices 4 and 5, arranged onthe sheet guide 3; and a double feed detection device 6 arranged betweenthe first transport device 4 and the second transport device 5 fordetecting a double feed of the sheets. A separating device separates thesheets stacked on the stacker 1 into a single sheet, and the first andsecond transport devices 4 and 5 feed the sheet to a processing position(platen 2). Predetermined processes such as reading images, printing,stamping or stapling are performed at the processing position. Then, thesheet is discharged to a discharge stacker 9.

The stacker 1 is composed of a tray for stacking the sheets. An emptysensor S1 for detecting the presence of sheets, and a size sensor S2 fordetecting a length of the sheets can be mounted according tospecifications required by the apparatus. The separating device forsequentially separating the uppermost or the lowermost sheets forfeeding is disposed at the leading end of the stacker 1.

A combination of a first transport roller 4 a and a friction pad 4 b asshown in the drawing, or a combination of a forward drive roller and areverse drive roller, or the combination of a feed roller and separatingclaw (corner separator) are widely known and used as the separatingdevice. Depending on the apparatus, even a vacuum separation can beused. The present invention allows for the use of either of thesemethods for separating sheets into single sheets. The drawing providedshows a configuration for the separating device that employs a firsttransport roller 4 a (although a belt is also perfectly acceptable) forrotating in a direction to feed the sheet, and a friction pad 4 b thatprevents a double feed of sheets. The transport device 4 transports theseparated sheet toward the platen 2. A resist roller 5 a for temporarilyidling a sheet in the transport path between the separating device andthe platen 2, and a transport roller 8 a are disposed for continuing totransport a sheet from the first transport roller 4 a to the platen 2.

At least two transport devices of the first and second transport devicebetween the stacker 1 and the processing platen 2 are provided. As shownin the drawing, the first transport roller 4 a is set as the firsttransport device; and the resist roller 5 a is set as the secondtransport device. The first and second transport devices 4 a and 5 a aredisposed with a spacing that is shorter than the shortest size sheetlength. Note that the resist rollers 5 employ a known configuration. Apair of rollers 5 a and 5 b in mutual contact forms a curve in sheetsfed from the transport roller 4 a to correct any skewing in the sheet.Then, at a predetermined timing, the rollers 5 a and 5 b feed the sheetto the platen 2.

A double feed detection sensor 6 and the sheet sensor 7 for detecting aleading edge of a sheet are arranged between the first and the secondtransport rollers 4 and 5. The double feed detection sensor 6 isconfigured by arranging a pair of a wave sending element 6 a and a wavereceiving element 6 b at opposite positions with the sheet movingtherebetween along the sheet guide 3. The sheet sensor 7 is configuredby arranging a pair of a light emitting element 7 a and a lightreceiving element 7 b to oppose each other. The double feed detectionsensor 6 shown in the drawing is an ultrasonic wave sensor. The wavesending element 6 a and the wave receiving element 6 b have a samestructure of a piezoelectric diaphragm. Note that the symbol 8 a in thedrawing represents a transport roller and is disposed at the sheet guide3 for controlling the feeding of the sheet to the platen at apredetermined speed.

The double feed detection sensor 6 is composed of an ultrasonic wavesensor. The wave sending element 6 a and the wave receiving element 6 bare disposed at opposite positions. Both the wave sending element 6 aand the wave receiving element 6 b are composed of a piezoelectricdiaphragm having a same structure, as shown in the example of FIG. 2. Ineach of the ultrasonic wave sensors, a piezoelectric diaphragm 11 suchas a piezoelectric diaphragm ceramic plate is embedded in and connectedto cylindrical external frame cases 10 made of a metallic material suchas an aluminum alloy. The case 10 is filled with a resilient plastic 13.Electrodes are formed on the front and back surfaces of thepiezoelectric diaphragms 11. One of the lead wires 12 is connected tothe piezoelectric diaphragm 11 and the other is connected to the case toprovide electrical grounding. When the high-frequency power is appliedto the lead wire 12, the piezoelectric diaphragm 11 oscillates at apredetermined frequency. Then, electromotive force generated by theexcited piezoelectric diaphragm 11 on one side is transmitted outsidealong the lead wire 12.

The wave sending element 6 a is electrically connected to a highfrequency power source. The following will describe the electricalconnection in reference to FIG. 3(a). The power 14 is connected to thehigh frequency oscillating circuit 15. The oscillating circuit generateshigh frequency voltages of between 30 KHz and 40 KHz. An amplifiercircuit 16 amplifies and supplies that to the wave sending element 6 a.When this occurs, the piezoelectric diaphragm 11, whose inherentoscillating frequency is set to a predetermined frequency, generatesultrasonic waves from the case 10 at that frequency. Note that theamplifier rate of the amplifier circuit 16 is set by the control CPU.Instruction signals from the CPU undergo D/A conversion and are thenrelayed to the amplifier circuit 16.

The ultrasonic waves generated from the wave sending element 6 a aretransmitted to the wave receiving element 6 b passing through the sheetS on the sheet guide 3. The ultrasonic waves that travel through thesheet cause the case 10 to oscillate in the wave receiving element 6 b,thereby causing the piezoelectric diaphragm 11 that is mounted to thecase 10 also to oscillate. The electromotive force generated by theoscillation of the piezoelectric diaphragm 11 is led to the lead wire 12from the electrode. The current is output as a detected valueproportional to the amplitude of the piezoelectric diaphragm 11.

The wave receiving element 6 b is connected to an amplifier circuit 18for amplifying the detecting current. The amplifier circuit 18 amplifiesthe detected current generated by the piezoelectric diaphragm 11. Theamplifier circuit 18 is connected to a smoothing circuit 19 composed ofan integrated circuit. The detected current of the amplified wave isaveraged by the smoothing circuit 19 and sent to the comparator circuit20. At the comparator circuit 20, the current from the smoothing thecircuit 19 is compared to a preset reference value. The reference valueis determined in the following way.

FIGS. 4(a) and 4(b) show the output values (analog voltage) of thesmoothing circuit 19. FIG. 4(a) shows the output value when a singlesheet is fed through the transport path. FIG. 4(b) shows the outputvalue when two sheets are fed. A sheet kicked from the stacker 1 istransported from the first transport roller 4 a to the second transportroller 5 a. In FIG. 4(a), the symbol A represents the output value whenthe leading edge of the sheet is moving from the first transport roller4 a toward the second transport roller 5 a. In that span of time, thewaveform is unstable. In the same drawing, the symbol B represents theoutput value when the sheet is nipped and held by the first transportroller 4 a and the second transport roller 5 a. During the span of time,the waveform is stable. In the same drawing, the symbol C is the outputvalue when the trailing edge of the sheet, still held by the secondtransport roller 5 a, is released from the first transport roller 4 a.At this time, the waveform becomes unstable again.

It is clear that the levels of the output values are different in theregion B representing the stable waveforms in the drawing when there isa single sheet or two sheets. Specifically, when the ultrasonic wavespass through the sheet, the degree of attenuation is smaller when thereis one sheet, which means there is a higher detected electrical current.Conversely, when there are two or more sheets, the degree of attenuationof the ultrasonic waves increases, thereby reducing the amount ofdetected electrical current. In other words, the detected current outputfrom the smoothing circuit is higher than the detected current whenthere is one sheet between the pair of transport rollers 4 a and 5 a,and it is lower than the detected current when there are two or moresheets when compared to the stable reference value. Note that thethicknesses of paper or the quality of paper used can differ. Therefore,it is necessary to experiment using a variety of different paper typesto find the appropriate reference value for of the machinespecifications.

Comparison data received from the comparator circuit 20 is thentransferred to the control CPU (control circuit) 21. Size sensors S2 andS3 arranged on the stacker 1, a sheet sensor 7, and a discharge sensor,not shown, arranged on the sheet guide 3 are connected to the controlCPU21. The sheet sensor 7 is arranged between the first transport roller4 a and second transport roller 5 a for transmitting the timing for thearrival of the leading edge of a sheet to the control CPU21.

The control CPU is connected for transmitting instruction signals to themotor driver circuit 22 of the drive motor M for driving the first andsecond transport rollers 4 a and 5 a. Power 25 is connected to a pulsegenerator 23 for supplying pulse currents to the motor driver circuit22. The drive motor M, which receives energy from the power 25, iscomposed of a stepping motor. The pulse generator 23 is connected to acounter 24. The counter 24 calls the number of pulse currents that aresupplied to the drive motor M, and is connected to the control CPU 21.

An operation for detecting a double feed of sheets on the apparatushaving the configuration of FIG. 1 will be explained with reference tothe flowchart in FIG. 7. The control programs on the CPU 21 areconfigured as described below for the transport control unit 28. Whenthe power 14 to the apparatus is turned on, the CPU 21 judges whetherthere is a sheet on the stacker 1 according to the status signalsreceived from the empty sensor S1 (F1). If a sheet is present, the CPU21 issues a start signal to the motor drive circuit 22. Receiving thesignal, the motor driver circuit 22 begins supplying pulse power fromthe power 25 to the drive motor M via the pulse generator 23. At thestart up of the drive motor M (F2), the first transport roller 4 a thatis connected to the drive motor M rotates in the clockwise direction ofFIG. 1 to kick a sheet from the top of the stacker 1.

The friction pad 4 b separates the sheets into a single sheet when thefirst transport roller 4 a kicks several sheets from the stacker 1. Thesingle sheet advances along the sheet guide 3, and the leading edge ofthe sheet arrives at the second transport device 5 passing between thedouble feed detection sensor 6, then the sheet sensor 7. The secondtransport device 5 is in a stopped state at this time. Therefore, whenthe leading edge of the sheet enters a nipping point (where the rollersare in contact with each other) between the second transport rollers 5 aand 5 b, the sheet forms a corrective loop shape. When the leading edgeof the sheet arrives at the sheet sensor 7, the transport control unit28 starts the timer (F3) by receiving the detection signal from thissheet sensor 7. The drive motor M stops after the time T1 (F4).

Next, the CPU 21 receives processing start signal from the mainapparatus such as an image reading device or printer as a paper feedsignal (F5), then restarts the drive motor M with that signal. Thedrawing shows a transmission mechanism configured of a one-way clutch,so that the forward drive of the drive motor M rotates the firsttransport roller 4 a and the reverse drive rotates the second transportroller 5 a (selectively).

Therefore, when the drive motor M restarts, it rotates the secondtransport roller 5 a, and the first transport roller 4 a remains in astopped status. The second transport roller 5 a then feeds the sheettoward the transport roller 8 a (F6). Simultaneous to this, thetransport control unit 28 of the CPU 21 restarts a timer T2 (F7). Thetimer T2 is set to a value where T1<T2, so that the loop in the sheet(curved in a registration loop) can be released. When a predeterminedamount of time passes for the timer T2, the CPU 21 issues a double feeddetection instruction signal (F8). Upon receiving the signal, thedetection signal/reference value comparator 29 inside the CPU 21receives the double feed comparison data (see FIG. 3(a)) and judgeswhether there is a double feed of sheets (F9). To detect a double feed(or judge the double feed), values detected from the wave receivingelement 6 a are compared with predetermined reference values (LVO inFIG. 2) at the comparator circuit 20. If the detected value is lowerthan the reference value, the system judges that there has been a doublefeed of two or more sheets. [0037]

Changes in the ambient environment or other conditions can causeerroneous detections to occur in the detection signal/reference valuecomparator unit 29 of the configuration shown in the drawings. Toprevent erroneous detections in the detected value of the wave receivingelement 6 b, the sheet is formed into a registration loop by the secondtransport device 5, and an air layer is formed between the two or moresheets. Then, the double feed detection is performed when theregistration loop is released. Also, the sheet is detected while nippedby both the first transport roller 4 a and the second transport roller 5a. As the sheet is being transported, the system judges with the averagevalue detected for a predetermined distance (length).

The detection signal/reference value comparison unit 29 executes adouble feed process (F10) when a double feed of sheets has been judged.In the double feed process, the apparatus stops and the operation paneldisplays that there has been a double feed in the system. In this case,an operator may either take a sheet out of the sheet guide 3 and resetit on the stacker 1, or discharge the sheet to a discharge stacker 9without any processing at the processing platen 2. When the detectionsignal/reference value comparison unit 29 has determined that thefeeding of a sheet is normal (that there has not been a double feed),the second transport roller 5 a and the transport roller 8 a feed thesheet to the processing platen where a predetermined process is appliedto the sheet (F12). When the trailing edge of the sheet passes over thesheet sensor 7, the CPU 21 detects the status signal to drive the drivemotor M (F2) to cause a next sheet to be kicked from the stacker 1 inthe same way.

Note that the transport roller 8 a is linked to a drive motor that isdifferent from the drive motor M mentioned above. However, it feeds thesheet to the processing platen 2 at a predetermined speed. The sheethaving undergone the predetermined process at the processing platen 2 issequentially fed and stored in the discharge stacker 9. A dischargesensor disposed on the edge of the sheet exit on the discharge stacker 9detects that a sheet has been stored (F13). At the status signal fromthe empty sensor S1 for detecting whether there are sheets on thestacker 1, the CPU 21 determines whether to continue the series in thejob or to end the series (Fl4). When the empty sensor S1 detects a nextoriginal on the stacker 1, the CPU 21 issues a paper feed instructionsignal (F5) to feed the next sheet to the processing platen 2.

The operation described above relates to conventional sheet feedingsteps. If the apparatus is a scanner device for reading imagessequentially on a sheet at the processing platen 2, the transportcontrol unit 28 which is executed in the CPU 21, described above,controls the speed of the drive motor M in the following way.

The transport control unit 28 sets the transport speed of the firsttransport device 4 and the transport roller 8 a according to the sheetprocessing conditions at the signals from the scanner device. Thetransport speed is determined according to the reading conditions ofimages by the scanner device. The conditions include color, orblack-and-white, high or low reading resolutions. Therefore, thetransport speed can vary according to the conditions. Generally, colorand high resolution readings are performed at a low speed, whereasblack-and-white and low resolution readings are set to a high speed.Therefore, the transport speed is set to a variety of high or low speedsaccording to the reading conditions.

FIG. 3(b) shows a sensitivity adjustment circuit (means) 35 embedded inthe CPU 21. The detective value from the wave receiving element 6 b isamplified by the amplifier circuit 18. The smoothing circuit 19 smoothesthis value, and the comparator circuit 20 compares it to the referencevalue. In this circuit configuration, first the amplifier rate istransmitted as an analog voltage value from the sensitivity adjustmentcircuit 35 of the CPU 21 to the amplifier circuit 16, which supplieshigh-frequency power to the wave sending element 6 a, via the D/Aconverter 17. The amplifier circuit 16 supplies high-frequency voltageto the wave sending element 6 a at the amplitude of the amplifier ratethat was set. On the other hand, detected currents from the wave sendingelement 6 a passes through the amplifier circuit 18, and the outputvalue (detected value) from the smoothness circuit 19 is converted intodigital values by the A/D converter 36. The values are then sent to thesensitivity adjustment circuit 35.

The sensitivity adjustment circuit 35 compares the output value of thewave receiving element 6 b to the predetermined reference value (at theoutput value/reference value comparator unit 37) and judges the outputvalue according to the complete results of the comparison (output valuejudgment unit 39). The amplifier rate is set (amplifier rate settingunit 38) according to the results of the judgment of the output valuejudgment unit 39. A gain is used to set the amplifier circuit 16 via theD/A converter 17.

A preset amplifier rate reference value 41 is read, for example, fromROM to set the amplifier circuit 16 gain to generate the ultrasonicwaves from the wave sending element 6 a. The wave receiving element 6 bdetects ultrasonic waves passing through a test sheet (reference sheet)and outputs that to the A/D converter 36 from the smoothing circuit 19via the amplifier circuit 18. The output value (an analog voltage value)is compared to the reference value. If the output value matches thereference value (in a constant range), the amplifier rate is stored inan amplifier rate memory 40. If the output voltage does not meet thereference value, the output value judgment unit 39 sets the amplifierrate, so that the output value (the analog voltage) rises topredetermined amount such as 0.1 V. (the amplifier rate setting unit 38)

The amplifier rate is increased by a predetermined amount, and theoutput value/reference value comparison unit 37 compares that again withthe output value of the wave receiving element 6 b. If they match, theamplifier rate is stored in the amplifier rate memory 40. If the outputvalue does not meet the reference value, the amplifier rate is increasedfurther by the predetermined amount. The same adjustments are repeated.If the amplifier rate setting reaches a maximum value 42 for the presetamplifier rate, the CPU judges that there has been a system error in thedouble feed detection of the wave sending element 6 a and the wavereceiving element 6 b. It may display an error message on the controlpanel for example to prompt an operator to repair or to continue thesheet processing on the system without using the double feed detectionfunction.

Note that according to the embodiment of the present invention, if theoutput value from the smoothness circuit 19 is within a range of 3.5V to4.0V, a correct (normal) amplifier rate is judged. Also, the referencevalue for adjusting the amplifier rate in the invention is set to avalue wherein the output value (an analog voltage value) after smoothingdoes not exceed a maximum of the minimum increment (for example 0.1 V)of an amp rte even when the element (or device) in use is combined witha highly sensitive component.

The following will explain adjusting the sensitivity of the amplifierrate on the apparatus of FIG. 1 while referring to the flow chart ofFIG. 6.

First, the sensitivity adjustment mode is entered (F20). The mode can beentered either by an operator using a switch on the control panels, orthe main apparatus such as an image reader or printer can automaticallyenter the mode at the same time as it is initializing the system. Whenentered, the CPU 21 judges whether there is a sheet on the stacker 1according to the status signals received from the empty sensor S1. Inthis case, the CPU 21 judges whether there is a test sheet placed on thestacker 1. If there is a test sheet, it starts the drive motor M.

The drive speed of the drive motor M is set to the same speed as whenprocessing a sheet, as described above. The sheet on the stacker 1 iskicked by the drive of the drive motor M. The leading edge of the sheetpasses through (F22) the ultrasonic wave sensor (double feed detectiondevice) to start the feeding of the test sheet (F23). When the leadingedge of the sheet arrives at the sheet sensor 7 (F24), the timer T1starts with a signal from the sheet sensor 7 (F25). After apredetermined amount of time has passed on the timer T1 (F26), the drivemotor M stops, thereby stopping the first transport roller 4 a (F27).

Next, the drive motor M drives in the reverse direction, therebystarting a drive of the second transport roller 5 a (F28). At the sametime that the drive motor M starts, the counter 24 connected to thepulse generator 23 counts the number of pulses. The CPU 21 uses this tojudge whether a predetermined length of the sheet has been transportedby the second transport roller 5 a. The predetermined amount for thesheet to be transported is set so that the sheet will be straightenedfrom the registration loop by the second transport roller 5 a.

Just about the time that the registration loop is released, thetransport control unit 28 of the CPU 21 stops the drive motor M ordecelerates it to a predetermined speed. The drive motor is stopped toprevent the variations in the ultrasonic sensor detection value causedby the movement of the sheet for the sensitivity adjustment that followsthereafter. The deceleration to a predetermined speed is set to anoptimum speed for the amount of time for the sensor adjustment thatfollows to be completed when changing the detection value according tothe transport speed and in the transport of the sheet. Normally, thespeed for sensitivity adjustment is set to a speed slower than theminimum speed for the predetermined process that occurs on a sheet atthe processing platen 2.

Sensitivity adjustments are conducted as described below while the sheetis traveling at a low speed or decelerating. First, the sensitivityadjustment circuit 35 supplies high frequency power to the wave sendingelement 6 a at the gain set by the amplifier rate reference value 41(F36). The output value (an analog voltage) from the wave receivingelement 6 b passes through the A/D converter 36 and is compared. If theresults match, the amp value is stored in the amplifier rate memory 40to complete the sensitivity adjustment. If the comparison results do notmatched, the output value judgment unit 39 increases the amplifier rateby the predetermined amount. When the amplifier rate setting unit 38does not exceed a maximum value (which is set to a range wherein thegain of the amplifier circuit is not saturated) of the amplifier rate,power is supplied from the amplifier circuit 60 to the wave sendingelement 6 a.

If the amplifier rate from the output value judgments unit 39 exceedsthe maximum value, the CPU 21 issues a failure signal to the mainapparatus (F38). The transport control unit 28 drives the transportroller 8 a at high speed to discharge the test sheet to the dischargestacker 9 (F39). Having been notified of the failure signal at the mainapparatus, the operator can then select whether to process the sheetwithout using the double feed detection function (the non-detectionoperating mode) (F40), or to repair the double feed detection device(F42) (by stopping machine operations). If the operator selects thenon-detection operating mode, a message indicating that it is possibleto feed the sheet but not to perform the double feed detection isdisplayed on the control panel. Then, the sheet can be processed asdescribed above (F41).

After adjustments are completed for the appropriate sensitivity from thewave receiving element 6 b (F32), the transport roller 8 a drives at ahigh speed to discharge the sheet on the sheet guide 3 into thedischarge stacker 9 (F33) The control panel of the main apparatusdisplays a message indicating that the adjustments are completed (F34).The adjustment mode is exited and the system is prepared for processinga next sheet (F35). Then, the appropriately adjusted amplifier rate isstored in the amplifier rate memory 40 and is used at the gain settingup of the amplifier circuit 16 when feeding a sheet.

An image reading apparatus according to an embodiment of the presentinvention will be explained next. FIG. 8 shows an image readingapparatus A and an image forming apparatus B mounted with the imagereading apparatus A as a unit. FIG. 9 shows a sheet feeding unit in theimage forming apparatus B.

The image forming apparatus B mounted with the image reading apparatus Ais embedded with a print drum 102 inside the casing 100; a paper feedcassette 101 that feeds paper to the print drum 102; a developer 108that forms images using toner on the print drum 102; and a fixer 104. Aprint head 103 such as a laser forms latent images on the print drum102. Paper fed from the paper feed cassette 101 is sent by the transportrollers 105 to the print drum 102, and the images formed by the printhead 103 are transferred to the sheet and then fixed thereupon by thefixer 104. The sheet with images is stored in the discharge stacker 121from the discharge roller 107.

The image forming apparatus B is widely known as a printer, and iscomposed of a paper feed unit, a printing unit, and a discharge storageunit. Their functions are various and are not limited to the structuredescribed above. For example, it is perfectly acceptable to employ aninkjet printer, or a silkscreen printer.

A data control circuit 109 is electrically interlocked to the print head103 to sequentially transfer image data that is accumulated by thememory apparatus 122 such as a hard disk for accumulating image data tothe print head. On the upper portion of the image forming apparatus B,the image reading apparatus A is mounted as a unit. The image readingapparatus A is mounted with the platen 112 on the casing 110. An opticalmechanism 114 and a photoelectric converting element 113 are arranged toread the original through the platen. A CCD is widely known and used forthe photoelectric converting element 113.

As shown in FIG. 9, the sheet feeding apparatus C is installed on theplaten 112. Above the platen 112 are arranged a paper feed stacker 115and a discharge stacker 116 above each other on the sheet feedingapparatus C. The sheets from the paper feed stacker 115 are guided tothe discharge stacker 116 via the U-shaped transport path 134 travelingover the platen 112.

Arranged on the paper feed stacker 115 are an empty sensor 117 fordetecting the sheets on the stacker, and a size sensor 132. As shown inthe drawing, a side guide 133 aligns the side edges of the sheets. Thesize sensor 132 and the side guide 133 are described in further detailbelow with reference to FIG. 10.

Arranged at a downstream side of the paper feed stacker 115 are aseparating roller 119 and a stationary roller 120 in contact with theroller. A kick roller 118 is mounted on the bracket 119 b mounted on therotating shaft 119 a of the separating roller 119. When the rotatingshaft 119 a rotates in the clockwise direction, the kick roller 118lowers to above the paper feed stacker 115. Conversely, when therotating shaft 119 a rotates in a counterclockwise direction, the kickroller 118 rises to a state shown in the drawing. The mechanism isdescribed in further detail below.

At a downstream side of the separating roller 119 are the double feeddetection sensor 123 that detects the double feed of the sheets, and asheet edge detection device 124 that detect the leading edge and thetrailing edge of the sheet. These are arranged in the transport path134. Also, equipped in order on the transport guide 134 are the resistrollers 125 a and 125 b; feed rollers 127 a and 127 b; a transportroller 129; and a pair of discharge rollers 130 a and 130 b. These aresequentially arranged to transport the sheets from the paper feedstacker 115 to the discharge stacker 116.

As shown in the drawing, a lead sensor 126 detects the leading edge ofthe sheet. A guide 128 supports the sheets at the platen 112 position. Acirculating path 131 circulates the sheets from the platen 112 to theresist rollers 125 a and 125 b through a path switching gate 131 a.

Next, the side guide 133 and the size sensor 132 will be explained. Apair of side guides 133 (133 a and 133 b) is disposed on the left andright of the paper feed stacker 115 to control the side edges of thesheets. The side guides are movably mounted in the width direction ofthe sheets. The racks 135 and 136 are integrally mounted to the left andright guides 133 a and 133 b. These mate with the pinion rotatably fixedto the paper feed stacker 115.

The left and right guides 133 a and 133 b are moved in the oppositedirections for the same amount by a pinion 137. The detection piece 139composed of a protrusion at a position that corresponds to the size ofthe sheets is disposed on one of the racks 136. The position of thedetection piece 139 is detected by the position sensor 138 mounted tothe bottom of the stacker 115. The position sensor is composed of aslidac volume and can detect the position of the side guide 133 bydetecting the variation in the resistance value varying with the lengthof engagement with the detection piece 139. Furthermore, size sensors132 are disposed in plurality on the stacker 115 to detect the trailingedge of the sheet.

The position sensor 138 detects the width direction of sheets on thestacker 115, and with the judgment by the size sensor 132 for sheetshaving the same width, the size of the sheet on the stacker 115 isdetected.

FIGS. 11(a) and 11(b) show a drive mechanism for the separating roller19 and the resist rollers 125. The paper feed drive motor 140 capable ofboth forward and reverse rotations drives the kick roller 118, theseparating roller 119, and the resist rollers 125. The transport drivemotor 141 drives the paper feed roller 127, the transport out roller129, and the discharge roller 130. With the forward rotation, the paperfeed drive motor 140 drives the kick roller 118 and the separatingroller 119. With its reverse drive, it drives the resist roller 125.Simultaneously, the paper feed drive motor 140 controls the rising andlowering of the kick roller 118. Force from the paper feed drive motor140 is transmitted to the resist rollers by a one-way clutch 142 via thebelts B1 and B2 only in one direction of rotation. At the same time, thepaper feed drive motor is connected to a rotating shaft of theseparating roller 119 by the one-way clutch 143 to transmit driverelatively with the one-way clutches 142 and 143.

The bracket 119 b is supported on the rotating shaft of the separatingroller 119 via the spring clutch 144. Drive is transmitted to the kickroller 118 mounted on the bracket 119 b by the transmission belt B3.When the paper feed drive motor 140 rotates in the forward direction,rotating drive is transmitted to the separating roller 119 and the kickroller 118. Simultaneously, the spring clutch 144 is released so thatthe bracket 119 b becomes free and lowers from an idled and raisedposition shown in FIG. 9 and the kick roller 118 touches the sheet onthe stacker. Rotating the paper feed drive motor 140 in the reversedirection transmits drive to the resist rollers 125. Simultaneously, thespring clutch 144 contracts, thereby raising the bracket 119 b to returnto the idled position shown in FIG. 9.

The transport unit drive motor 141 is connected to the feed rollers 127,transport rollers 129, and discharge rollers 130 via the belts B5, B6and B7. The feed rollers 127 and transport rollers 129 always rotate inone direction with the forward and reverse rotations of the motor withthe one-way clutch. The discharge rollers 130 rotate forward and reversewith the forward and reverse rotations of the motor.

Sensors for detecting the leading edge of the sheets are arranged in thetransport path 134. Their functions will be explained. The size sensors132 that detect the size of the sheets set on the paper feed stacker 115are arranged in plurality. These detect the size of the sheets tocontrol sheet transport. The empty sensor 117 is disposed on the leadingedge of the paper feed stacker 115 to detect the sheets on the stacker.This detects the transport of the final sheet and sends a signal to theprocessing apparatus, such as the image reading apparatus A. At adownstream side of the separating roller 119 are disposed the doublefeed detection sensor 123 described above and the sheet edge detectionsensor 124.

A lead sensor 126 is disposed in front of the paper feed roller 127.This relays the leading edge of the sheet to the image reading apparatusfor reading images and calculates the starting line for printing.Simultaneously, if the sheet is not detected after a predeterminedamount of time from the paper feed instruction signal from the resistroller 125, the drive motor stops because of a jam and issues a warningsignal. At a downstream side of the transport rollers 129 is disposedthe discharge sensor 145 to judge jams by detecting the leading-edge andthe trailing-edge of the sheets.

The following will outline an operation of the apparatus describedabove. The power to the apparatus is turned on, and sheets are placed inthe paper feed stacker 115. By setting the sheets, the empty sensor 117detects the sheets and starts the paper feed drive motor 140. With therotation of the paper feed drive more 140, the kick roller 118 andseparating roller 119 separate the sheets and kick them out. They arefed to the transport guide 128 between the separating roller 119 and thetransport rollers 125. The sheet edge detection means 124 (hereinafterreferred to as sensor 124) detects the leading edge of the sheets(ST101). The timer T1 activates after the detection signal of theleading edge of the sheet (see FIGS. 4(a) and 4(b)) to stop the motor140 after a predetermined amount of time (ST102).

According to the operations as shown in FIG. 12(a), the sensor 124detects the leading edge of the sheet and activates the timer T1. Next,in FIG. 12(b), the leading edge of the sheet strikes the resist rollers125 and a loop is formed in the sheet. In this state, a set amount oftime for the timer T1 ends and the motor 140 stops.

When the paper feed instruction signal is generated from the controlunit of the image reading apparatus A, the motor 140 starts rotatingagain in the reverse direction. Also, with the paper feed instructionsignal, the timer T2 is activated. With the timer T2, the registrationloop is removed and the sheet is supported between the separating roller119 and the resist rollers 125 for transport in a straight line as shownin FIG. 11(c).

Next, as shown in FIG. 11(d), until the trailing edge of the sheet isreleased from the separating roller 19, the double feed detection sensor123 detects the double feeding of the sheets. The trailing edge of thesheet transported in that way is detected by the sensor 124.Approximately about the time when the trailing edge of the sheet isdetected, the lead sensor 126 detects the leading edge of the sheet andthe feed roller 127 feeds the sheet toward the platen 112.

When the leading edge of the sheet is detected by the lead sensor 126and the sheet reaches the platen 112, the reading process is executed aselectrical signals by the optical mechanism 114 and the photoelectricconverting element 113. After the sheet has been read, it is dischargedto the discharge stacker 116 by the transport rollers 129 and thedischarge rollers 130. The discharge of the sheet is detected by thedischarge sensor 145.

The double feed detection device is composed of the ultrasonic wavesensor arranged in a path leading to the resist rollers 125 at adownstream side of the separating roller 119 (feeding device). Thesensitivity adjustment circuit 35 explained with reference to FIG. 3(a)is disposed on the wave sending element of the ultrasonic wave sensor. Asmoothed output value (analog voltage) is compared to a reference value.If a test sheet is placed on the stacker 115 and the sensor sensitivityadjustment mode is selected using the control panel of the apparatus,the output of the wave sending elements is adjusted to the appropriateconditions as described in the operation flowchart shown in FIG. 7.

The disclosure of Japanese Patent Application No. 2004-170395, filed onJun. 8, 2004, is incorporated in the application.

While the invention has been explained with reference to the specificembodiments of the invention, the explanation is illustrative and theinvention is limited only by the appended claims.

1. A sheet feeding apparatus comprising: a stacker for stacking sheets;a feeding device for feeding the sheets on the stacker to apredetermined processing position; a drive device for driving thefeeding device; a double feed detection device arranged at a downstreamside of the feeding device for detecting a double feed of the sheets andhaving a sending element and a receiving element; a control device forcontrolling a transport speed of the feeding device; and a sensitivityadjustment device for comparing a detected value of the receivingelement with a predetermined reference value for adjusting an output ofthe sending element, said sensitivity adjustment device adjusting theoutput of the sending element in a condition such that the controldevice controls the drive device to stop the sheet or to decelerate thesheet at a predetermined speed.
 2. A sheet feeding apparatus accordingto claim 1, wherein said sending element includes an ultrasonic wavesending element and said receiving element includes an ultrasonic wavereceiving element, said sensitivity adjustment device having anamplifier device for increasing or decreasing amplitude of theultrasonic wave sending element.
 3. A sheet feeding apparatus,comprising: a stacker for stacking sheets; a feeding device forseparating the sheets on the stacker into a single sheet and feeding thesheet to a predetermined processing position; first and second transportdevices arranged between the stacker and the sheet processing positionwith a predetermined distance therebetween; a double feed detectiondevice arranged between the first and the second transport devices fordetecting a double feed of the sheets, and having a sending element anda receiving element disposed oppositely; a control device forcontrolling transport speeds of the first and second transport devices;a sheet sensor for detecting that the sheet fed from the stacker reachesa downstream side of the first and second transport devices; and asensitivity adjustment device for comparing a detected value of thesending element with a predetermined reference value for adjusting anoutput of the sending element so that the sensitivity adjustment deviceincreases or decreases the output of the sending element according to asheet leading edge detection signal from the sheet sensor.
 4. A sheetfeeding apparatus according to claim 3, wherein said control devicestops the drive device according to the sheet leading edge detectionsignal from the sheet sensor, and restarts the drive device after thesensitivity adjustment device increases or decreases the output of thesending element.
 5. A sheet feeding apparatus according to claim 3,wherein said control device drives the drive device at a predeterminedspeed according to the sheet leading edge detection signal from thesheet sensor, and drives the drive device at a speed higher than beforeafter the sensitivity adjustment device increases or decreases theoutput of the sending element.
 6. A sheet feeding apparatus according toclaim 3, wherein said first and said second transport devices have afirst operation mode to transport the sheet to the sheet processingposition at a first speed to perform a specific process, and a secondoperating mode to transport the sheet to the sheet processing positionat a second speed without performing the specific process, said firstspeed being higher than the second speed.
 7. A method of detecting adouble feed of sheets transported from a stacker, comprising: atransport step for transporting the sheet on the stacker to apredetermined processing position; a double feed detection step arrangedbetween the stacker and the sheet processing position for detecting thedouble feed of the sheets with an ultrasonic wave sensor having asending element and a receiving element; a sensor sensitivity adjustmentstep for changing an output of the sending element while the sheet isstopped or is decelerated at a predetermined speed between the stackerand the sheet processing position; and a discharging step fordischarging the sheet by restarting the sheet or moving the sheet at aspeed higher than the predetermined speed after the sensor sensitivityadjustment step.
 8. A method according to claim 7, wherein said sensorsensitivity adjustment step is executed when the sheet is nipped by atleast two transport devices arranged at an upstream side and adownstream side in a direction that the sheet is transported.